Solo supernovae challenge cosmic distance standards

Some supernovae might run a short fuse. These exploding white dwarf stars were all thought to detonate with the same energy when they reached a particular size – with a little nudge from a companion star. But new research suggests some of them may go solo.

Type Ia supernovae go off regularly because they are thought to be runaway thermonuclear reactions inside white dwarfs, the remnants of normal stars that shed their outer layers of hydrogen and helium, leaving an ultra-dense carbon and oxygen core. Carbon fuses with oxygen and itself, and the bomb-like energy this releases then blows the dwarf apart.

To be dense enough to start this kind of reaction, white dwarfs need a mass 1.4 times that of the sun. That being so implies all type Ia supernovae have the same brightness. This makes them useful as “standard candles” to calculate cosmic distances and the effect of dark energy.

But white dwarfs don’t grow that big on their own. “Everybody always thought that you had to have two white dwarfs or a dwarf and another star,” says Susana Deustua, a scientist with the Supernova Cosmology Project. To reach the threshold, astronomers thought they either steal mass from or crash into their neighbours.

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Trigger happy

But there’s a problem&colon; observations show that half of the type Ias come from dwarfs that are not technically massive enough to explode, says Marina Orio at the University of Wisconsin-Madison.

According to a model built by Orio and her colleagues, some singleton white dwarfs are trigger happy. Lighter elements like hydrogen and helium can cause a runaway explosion at a lower density than carbon alone, so if a dwarf had any left over, it could explode sooner. They calculate that a “contaminated” white dwarf could go supernova at just 0.85 solar masses, even without a companion (arxiv.org/abs/1409.1104).

These smaller supernovae will release less energy and appear dimmer, making astronomers assume they are further away than they actually are. These findings could mean that up to half of supernovae are up to 30 per cent closer than believed. It could also mean dark energy is more powerful than estimated from previous calculations involving type Ias.

“This study gives us an understanding of the variety and diversity of the supernovae,” says Deustua. This new zoo of type Ia supernovae might all be the same species, but they’re different beasts.